Rit (Vísindafélag Íslendinga) - 01.06.1967, Síða 41
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Iceland, but comprise distinct dyke swarms. Gibson takes the
dyke swarms to indicate that volcanic activity was confined
to a narrow area at each time, where an elongated lens-like
pile of lavas accumulated over a dense dyke swarm (Fig. 1).
Periodically the volcanic zone shifted westwards, where a
new lenticular pile of lavas was piled up on the flanks of
the older lenses. One condition for the validity of this hypo-
thesis is both up- and down-dip thinning of the lensoidal
lava piles. Walker has earlier demonstrated up-dip thinning
and Gibson names two examples of a group of lavas which
thin down-dip also, but concludes that generally less than
half of each lens is exposed above sea level, thus accounting
for the scarcity of down-dip thinning. He estimates that 0.6
—1.8 km thickness of each lens is exposed, whereas 0.8—2.0
km thickness is concealed below sea level, giving 2—4 km as
the total thickness of the basalt pile (Fig. 1).
The chief advantage of Gibson’s hypothesis is how well it
agrees with seismic data on the thickness of the basalt pile
(layers 1 and 2 of Pálmason, 1963). But the hypothesis also
raises a number of questions. Consider first formation of a
basalt pile by growth of lensoidal basalt lava groups, where
each new lens is formed on the western flank of the older
ones. Each growing lens thus rests on the sloping western
flank of an older lens, and consequently the new lens will
be asymmetrical, with the western part much longer than
the eastern one. Taking this asymmetry of the lenses into
account, the western limb of each lens, i.e. the thickness con-
cealed down-dip and therefore the thickness of the basalt pile
should be very much greater than assumed by Gibson. Gib-
son accounts for the prevailing westerly dip in eastern Ice-
land as sagging of older lenses under the load of younger ones
into the “modified mantle” layer. The tilting therefore re-
quires a higher position of the “modified mantle”/basalt con-
tact in the central active zone, where sagging has not taken
place, than outside it. Gibson’s hypothesis therefore demands
a rise in the “modified mantle” under the active zone, but no